(i) Field of the Invention
The present invention relates to a combustible gas sensor for detecting a combustible gas contained in a gas to be measured. The present invention also relates to a method for detecting the deterioration of a catalyst for cleaning the exhaust gas of an automobile.
(ii) Description of the Related Art
A combustible gas sensor intends to detect a combustible gas contained in a gas to be measured, such as a gas generated at combustion or an exhaust gas, and it has been required to quantitatively determine the concentration of the combustible gas by the sensor. The exhaust gas is exhausted from, for example, an internal combustion engine, an external combustion engine or a combustion furnace which utilizes heavy oil, gas oil, gasoline, a natural gas or the like as a fuel.
In the combustible gas sensor, the so-called contact combustion type is employed, and in this type, the combustible gas is burned with the aid of a platinum catalyst and a temperature raised by the combustion heat is then measured. In a conventional contact combustion type combustible gas sensor, an oxidation catalyst of a noble metal such as Pt, Pd or Rh is supported on beads formed by sintering alumina in a porous state on a platinum coil. The platinum coil is heated by an external heater up to about 300.degree. C. and then brought into contact with the gas to be measured, so that combustion occurs to raise the temperature of the platinum coil. The thus raised temperature is then detected as a change of the electric resistance of the platinum wire buried in the beads by means of a bridge circuit.
The conventional combustible gas sensor, however, is constituted so that the combustible gas may come in contact with the platinum resistor, and therefore when the sensor is used at a temperature as high as 900.degree. C., the resistance value of the platinum resistor changes inconveniently. Thus, the conventional combustible gas sensor has a drawback that its use at a high temperature is impossible. In addition, when the temperature of the gas to be measured changes remarkably, for instance, when it changes from room temperature to about 900.degree. C., there is no way to accurately measure the temperature rise attributable to the heat generation of the combustible gas contained therein.
On the other hand, various combustible gas sensors using oxide semiconductors have also been investigated. However, the oxide semiconductor type combustible gas sensor has a disadvantage that its performance is vitally affected by oxygen, humidity or the like, and another disadvantage that the resistance value of the semiconductor changes at a high temperature.
Heretofore, there has been researched a method for detecting the deterioration of a catalyst such as an exhaust gas cleaning catalyst which intends to eliminate the combustible gas. For instance, Japanese Utility Model Application Laid-open No. 61919/1987 has suggested a method for detecting the deterioration of a catalyst which comprises arranging temperature sensors on the upstream side and on the downstream side of the catalyst, respectively, and then comparing a temperature of the exhaust gas on the upstream side with that of the exhaust gas on the downstream side. In this method, the deterioration of the catalyst is required to be judged after an automobile has run at a constant speed of 40 to 60 km/hr for several minutes. This constant running is necessary to thermally stabilize an exhaust system, because the catalyst has a large heat capacity. Thus, in order to improve the detection accuracy of the catalyst deterioration, the automobile is required to run at the constant speed for a further long time.
However, under actual running conditions of the automobile where acceleration and deceleration are repeated, the above-mentioned requirement of the running at the constant speed can hardly be met, and therefore it is difficult to detect the deterioration of the catalyst with a high precision. Furthermore, in this method, a sufficient temperature difference cannot be obtained unless the temperature sensors are inserted into an exhaust tube so that they may be close to the central axis of the exhaust tube, and hence this method has a drawback that the pressure of the exhaust gas is increased and the output power of the engine is consequently reduced. In addition, the two temperature sensors are required, which makes the system complicated, with the result that cost increases inconveniently.
When the combustible gas sensor is used in the exhaust system of the automobile, the output of the sensor element is connected to an electronic device other than the sensor element, a central processing unit or the like, and the temperature or the like is detected by the electronic device, the central processing unit or the like. Here, a resistance value R of a resistor having a positive resistance temperature coefficient is represented by R=R.sub.0 (1+at-.beta.t.sup.2) wherein t is a temperature, and R.sub.0 is a resistance value of the resistor at 0.degree. C., but it is not limited to the resistance value at 0.degree. C. and it may be the definite resistance value of the resistor at a certain temperature.
Accordingly, in order that the electronic device, the central processing unit or the like measures the temperature or the like, it is necessary that the resistance value at the certain temperature of the resistor in the specific sensor element connected to the electronic device or the like should be previously input to the electronic device or the like. Furthermore, when the sensor elements are attached to the automobiles on such a mass production line as in an assembly plant of the automobiles, the resistance value of the resistor in each sensor element is required to be promptly put to the electronic device or the like. For example, it is impractical that the resistance value of the resistor is input to the computer of each automobile by way of a keyboard.
However, in the process of manufacturing the sensor elements, the resistance value of the resistor may inevitably scatter to some extent. For example, the resistor can often be formed by printing its pattern on the surface of a ceramic green sheet and then baking the green sheet having the resistor pattern. The resistance value of the thus formed resistors typically has a deviation of .+-.10%.
Japanese Patent Application Laid-open No. 279831/1992 has described a technique of trimming the resistor by means of laser irradiation in order to minimize the deviation in the resistance values of the resistor. However, when the resistor is trimmed by the laser irradiation, the temperature of the resistor rises. Since the resistor has a large resistance temperature coefficient, it is difficult to heighten the precision of the resistance value of the resistor, so that the resistance value of each sensor element largely scatters sometimes.